Referring to FIGS. 8 to 11, a conventional shelf comprises a plurality of shelf boards (40) which are parallel with each other. Each of shelf boards (40) has a board (not shown in FIGs.), and a frame (41) is formed around the board. Moreover, the shelf has at least four posts which are formed at four corners thereof and configured to connect the shelf boards (40) through a plurality of plastic connecting units. Each of the frames (41) has two horizontal folded portions (411) respectively formed at an upper edge and a lower edge thereof, and each of the folded portions (411) is formed with a preferred width. Each of the folded portions (411) has four recesses (412) respectively formed at four corners thereof, and four sleeves (42) are respectively engaged at four corners of the frame (41) through the recesses (412) at the same corner. Through ultrasonic welding, the sleeves (42) are configured to be firmly secured at the recesses (412) of the frame (41). In the assembly process, each of the four posts is configured to be inserted into and secured inside the sleeve (42) of the shelf board (40) through the plastic connecting unit, and wherein each of the plastic connecting units is configured to be abutted against the sleeve (42) and firmly secured on the post, thus completing the connection between the shelf board (40) and the posts.
However, the conventional shelf is disadvantageous because: although the sleeves (43) are respectively and firmly secured inside the recesses (412) of the frame (41) of the shelf board (40) through the ultrasonic welding, each of the sleeves (43) only contact the folded plates (411) of the frame (41) at two end edges thereof, which only has small contact areas between the recesses (412) and the single sleeve (43). When a user put an object on the shelf board (40), the shearing strength induced at positions between the recesses (412) and the sleeve (43) is configured to break the structure of the shelf board (40), which is unsafe for use. Therefore, there remains a need for a new and improved design for a shelf to overcome the problems presented above.